It is recently practiced to produce cylindrical fan wheels from synthetic resin by injection molding as disclosed in Japanese patent publication No. 48043/1972. To render the cylindrical fan wheel easy to assemble, the fan wheel comprises molded units joined together end-to-end in the axial direction to a specified length and each including a support disk and a predetermined number of blades arranged on one or both sides of the disk.
With such construction, however, the unit necessitates a complex and expensive die for molding, and the die must have such draft that the width and thickness of the blade will decrease progressively from its base portion to its end, as required by the injection molding operation. When the blades are thus longitudinally tapered, rotation of the fan wheel permits part of the air current to flow in the axial direction, entailing a pressure loss, and the fan wheel has another drawback that the joints between the units have low strength against vibration and other stress, especially against impact.